An electrical generator typically operates by rotating a coil of wire relative to a magnetic field (or vice versa). In modern electrical generators, this magnetic field is typically generated using electromagnets known as field coils. An electrical current in these field coils provides the magnetic field necessary to induce an electrical current in the main generator coil. The current and voltage output of the main generator depends on the current in the field coils. Thus, as the generator load changes, the magnetic field strength must be adjusted in an attempt to maintain constant generator output. This is achieved by regulating the current in the field coils. Thus, it is desirable to regulate continuously the current in the field coils.
In many modern electrical generators, the electrical current in the field coils is provided by an exciter generator. Thus, the problem of regulating the main generator field strength is typically achieved by regulating the output of the exciter generator. The exciter generator can include its own exciter field coils, in which case the output of the exciter generator can be controlled by regulating the current supplied to the exciter field coils.
The output of a voltage regulator can provide power to the field coils of an exciter or generator. The voltage regulator can therefore be used to control the output of an exciter generator, and thereby control the output of a main generator. One important purpose of a voltage regulator is to maintain the output of a main generator at a constant voltage, known as the “set point,” under various conditions. The difference between the set point and the actual main generator output is “error.”
Many different types of voltage regulators have been proposed for excitation systems. One of the most popular modern voltage regulator systems utilizes a Proportional, Integral, and Derivative (“PID”) controller. A PID controller monitors the output of the generator and adjusts its own output depending on sensed generator output. As the name implies, the PID controller provides three types of control: proportional, integral, and derivative. Proportional control responds in proportion to the error. Integral control responds to the sum of previous errors. Derivative control responds to the rate of change of the error.
The relative weights of these three types of control in a PID controller must be set for accurate control of the generator output. Choosing these relative weights is known as “tuning” the PID controller. The goal is to achieve a “fast” excitation system that will respond to a disturbance by quickly bringing the generator output back to the set point. An ideal excitation system achieves the set point with as little overshoot and undershoot as possible. Overshoot occurs when the controller provides too much current, thereby causing a “spike” in main generator output. Undershoot occurs when the controller provides too little current, thereby causing a “dip” in main generator output. A poorly tuned PID controller will result in poor performance, e.g., overshoot, undershoot, or slow response time. A well-tuned excitation system offers benefits in overall operating performance by responding quickly to transient conditions such as system faults, disturbances, and motor starting. After a fault, a fast excitation system will improve transient stability by holding up the system voltage and providing positive damping to system oscillations. A well-tuned excitation system will minimize voltage overshoot after a disturbance and avoid the nuisance tripping of generator protection circuits. When a motor is powered by a generator, the motor presents a large load while the motor is starting, which can cause the generator output voltage to dip. A dip in generator output voltage can cause damage to the motor as the motor will increase its current consumption and heats up due to resistive heating within the motor. During motor starting, a fast excitation system will minimize the generator voltage dip and reduce the heating losses of the motor.
The controller gains are determined using several excitation system parameters, such as voltage loop gain, open circuit time constants, and so forth. These parameters vary not only with the system loading conditions, as generally illustrated by numeral 2 in FIG. 1, but also system configuration dependent gains such as power input voltage.
In general, since the calculation of loop gain requires several excitation system parameters that are generally not available during commissioning, e.g., specifically the machine time constant, this lack of information increases commissioning time. Many times there is no access to the actual equipment but only to a manufacturer's data sheet, or some typical data set. For this case, the only available measurement to check excitation system performance is the combined response of exciter and generator as generally illustrated by numeral 4 in FIG. 2. Under these conditions, commissioning a new voltage regulator becomes a challenging task.
One method of tuning the PID controller is by trial-and-error. Trial-and-error is tedious and adds significantly to commissioning time. Consequently, several automatic “self-tuning” algorithms have been proposed.
One difficulty in regulating the voltage output of a generator arises due to the inductive properties of a coil of wire, such as a field coil winding. Since the excitation system and the generator contain inductive coils, there is a time delay between a change in output voltage from the voltage regulator and the corresponding change in generator output voltage. The length of this delay is determined by “time constant.” The main time constants of concern are the exciter time constant and the generator time constant. In the present invention, these time constants are estimated and taken into account when tuning the PID controller, thereby achieving improved performance. Also, today's digital voltage regulators are designed to achieve about 0.25% regulation accuracy at rated voltage. The accuracy of a digital voltage regulator is mostly determined by truncation error in an analog/digital (“AD”) converter and thermal noise in interface circuits. Institute of Electrical and Electronics Engineers, Inc. (“I.E.E.E.”) Standard 421.5 recommends two percent (2%) step responses for testing or analyzing performance of an excitation control loop. Thus, generator voltages due to small perturbation in excitation are measured with a very poor signal-to-noise ratio. For example, the relative accuracy of a two percent (2%) step response test becomes about a ten percent (10%) error in measurement. Therefore, in this case, it is difficult to identify the exciter and generator time constants. Experiments show very slow convergence speed in identification, which is much slower than today's fast excitation system requirements.
The present invention is directed to overcoming one or more of the problems set forth above.